AlertDialog.qml

import QtQuick 2.10
import QtQuick.Controls 2.3
import QtQuick.Window 2.2

Dialog {
    id: _alertDialog

    property real extraHeight: 0
    property real baseWidth: 300

    x: parent ? (parent.width - width) / 2 : 0
    y: parent ? deltaY() : 0
    width: parent ? Math.min(Math.max(implicitWidth, baseWidth), parent.width - 24) : implicitWidth
    height: parent ? Math.min(implicitHeight, parent.height - 24) : implicitWidth
    modal: true
    focus: true

    function deltaY() {
        var unscaled = Qt.inputMethod.keyboardRectangle.height / Screen.devicePixelRatio;
        var availHeight = (parent.height + extraHeight) - unscaled - 24; //margins
        var rawDelta = (Math.max(0, availHeight - height) / 2);
        return rawDelta + 12 - extraHeight; //spacing
    }
}